Method for plasma separation and measurement

ABSTRACT

The system for transferring plasma to a test substrate comprises an applicator and the test substrate. The applicator includes a blood reservoir and a filter element which separates cellular components from the blood as plasma is passed to the test substrate. The test substrate includes an absorptive element which receives the plasma from the applicator. In one embodiment, measured amounts of plasma are transferred when the absorptive element becomes saturated, thus stopping the flow of plasma. In another embodiment, excess blood and cellular components are removed from the measured plasma by disengaging the applicator from the test substrate. In another embodiment, plasma transferred to the absorptive element is eluted from the absorptive element by subsequent application of an elution medium.

This is a Division of application Ser. No. 08/326,974 filed Oct. 21,1994, now U.S. Pat. No. 5,589,399 the disclosure of which isincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems and methods forassaying biological samples. Particularly, the present invention relatesto a system and method for separating plasma from whole blood andtransferring measured amounts of the plasma to a test substrate.

Many diagnostic markers and other analytes are best determined frompatient blood samples. The ability to measure such markers and analytesdirectly in blood, however, is problematic because of the presence ofblood cells, particularly red blood cells, in whole blood. Thus, amajority of analytical systems and methods intended for measuring bloodmarkers and analytes rely on detection in a plasma or serum sample, notin a whole blood sample.

A wide variety of blood separation systems and methods have beendeveloped over the years particularly for use in performing plasma andserum assays. Such systems may be generally categorized as centrifugal,chemical, and filtration. Centrifugal systems rely on centrifugation toremove cellular components from the blood. Chemical systems rely onantibodies, lectins, or the like, for binding and removing cellularcomponents. Finally, filtration relies on chromatographic and/orsize-exclusion filtration elements for separating the cellular bloodcomponents from the resulting plasma fraction.

The present invention is concerned in particular with filtration systemsand methods for producing plasma from whole blood prior to performing anassay on the plasma. Heretofore, filtration elements have often beenconstructed integrally in assay devices, where blood is applied to thefilter element and plasma is separated and flows to a reaction zone,usually without precise measurement of the plasma and without theability to separate the excess blood and cellular blood components fromthe assay device. Such prior assay devices were often limited in theirability to provide diluents, eluents, and other liquid reagents in themeasurement system. Furthermore, such prior devices were frequentlylimited to a single reaction zone for determining a single marker oranalyte.

For these reasons, it would be desirable to provide improved systems andmethods for separating plasma from whole blood for use in assays andassay devices. It would be particularly desirable if such systems andmethods could provide for accurate and precise (receptacle) measurementof the plasma being separated as well as for direct transfer of themeasured plasma volume to a test substrate. It would be furtherdesirable if systems and methods allowed for complete separation of theexcess blood and cellular components from the separation device, as wellas for subsequent elution of the separated plasma to one or moreassociated reaction zones. In particular, it would be desirable if thesystems and methods of the present invention provided for assaysemploying a plurality of reaction zones capable of measuring amultiplicity of analytes from a single blood sample, particularly from alow volume blood sample of 500 μl or below.

2. Description of the Background Art

Filter elements for separating plasma from whole blood in combinationwith absorptive pads are shown in a number of patents, including U.S.Pat. Nos. 5,262,067; 5,132,086; 5,110,724; 4,987,085; 4,933,092;4,816,224; 4,753,776; 4,477,575; and 3,663,374. In particular, U.S. Pat.No. 5,132,086 shows a device where sample is absorbed into a filteringpad, optionally by dipping the pad into the sample to obtain asemiquantitative volume of sample, and sample then allowed to flowthrough a reaction zone on an adjacent absorptive strip. U.S. Pat. No.5,110,724 shows a device for separating plasma from whole blood anddistributing the plasma over an absorptive distribution strip. Theplasma is then transferred to a plurality of separate reaction pads bysimultaneously contacting the pads with the distribution strip. U.S.Pat. No. 4,987,085 shows a device having a cell-filtering structureformed over an absorptive pad. Blood is applied to the filteringstructure, and the filtering structure removed after plasma has beenabsorbed in the pad. U.S. Pat. Nos. 4,816,224 and 4,477,575 describe adevice having a cell-filtering structure removably formed over anabsorptive reaction layer. Other patents showing assay devices includingseparation membranes and absorptive pads include U.S. Pat. Nos.4,678,757; 4,952,516; 4,999,163; 5,000,922; 5,135,719; 5,186,843;5,266,219; and 5,308,483.

SUMMARY OF THE INVENTION

According to the present invention, systems and methods are provided forseparating plasma from whole blood and simultaneously transferringmeasured amounts of the separated plasma to a test substrate. Thesystems and methods are capable of delivering very precise and accurateplasma volumes to the test substrate, typically within 15% andpreferably within 5% of the nominal value, even from very small wholeblood samples, typically below 500 μl and often below 50 μl. Theseparated cellular components and excess whole blood may be completelyremoved from the test substrate, and an eluent subsequently applied tothe test substrate to distribute the plasma to an associated reactionzone, usually on the test substrate. Frequently, a plurality of reactionzones will be located on the test substrate, permitting simultaneousdetection of a multiplicity of analytes. The ability to initiallyprovide such accurate and precise plasma volumes, as well as tosubsequently elute the measured plasma to associated reaction zone(s),permits accurate quantitative measurement of target markers and analyteseven when the initial blood sample is very small and the separatedplasma is highly diluted.

A system according to the present invention for transferring measuredamounts of plasma from unmeasured whole blood samples to a testsubstrate comprises an applicator and a plasma-receiving test substrate.The applicator includes a blood-receiving reservoir which is in fluidcommunication with a plasma transfer port. A filter member extendsacross the plasma transfer port, where the filter member permits thepassage of plasma and plasma macromolecules but inhibits the passage ofblood cells. Thus, plasma is separated from the blood in the reservoiras the plasma advances through the filter member under capillary forces,as described in more detail below.

The plasma-receiving test substrate includes an absorptive elementhaving a contact surface configured to engage the plasma transfer portof the applicator. The absorptive element, which may be a fibercapillary matrix or a molded capillary matrix, will usually have aplasma capacity which is less than the plasma capacity of the bloodreservoir in the applicator. Thus, plasma transfer from the applicatorwill terminate as soon as the plasma has saturated the absorptiveelement, resulting in the transfer of a very precise and accurate volumeof plasma to the test substrate. Usually, the applicator will be removedfrom the test substrate leaving the transferred plasma volume availablefor subsequent testing, free from cellular components and excess blood.

In a preferred aspect of the system of the present invention, theapplicator further includes a tube for drawing blood into theblood-receiving reservoir. Optionally, the tube may be a capillary tubeor needle suitable for pricking a patient's finger to draw a small bloodvolume e.g. hypotube. Alternatively, the tube may be a needle suitablefor penetration through the plenum or stopper of a blood collectionvessel in which patient blood has previously been collected, e.g. avacuum blood collection device. Usually, the blood-receiving reservoirwill have a volume in the range from 50 μl to 500 μl, and the plasmatransfer port will have an area in the range from 15 mm² to 200 mm². Thefilter member in the applicator will also preferably have a crownedcross-sectional profile to enhance contact with the contact surface ofthe absorptive element on the plasma-receiving test substrate.

In a further preferred aspect of the system of the present invention,the absorptive element on the test substrate will have a contact area inthe range from 10 mm² to 200 mm² and a plasma capacity below 200 μl,preferably in the range from 1 μl to 100 μl. The plasma-receiving testsubstrate optionally further includes a fluid-receiving surface disposedadjacent to the absorptive element so that plasma eluted from theabsorptive element will be received on the surface. The plasma-receivingtest substrate may further include at least one reaction zone disposedon the fluid-receiving surface to permit reaction of the eluted plasmafor the detection of a target marker, analyte, or the like. Preferably,the test substrate will include a plurality of reaction zones,optionally arranged on a disk having the absorptive element at itscenter.

A first method for transferring plasma from unmeasured blood samples toa test substrate comprises applying a volume of whole blood to anabsorptive element through a filter membrane. The filter membranepermits the passage of plasma and plasma macromolecules but inhibits thepassage of blood cells so that blood cells are retained above the filtermembrane. An elution medium is subsequently applied to the absorptiveelement to elute the separated plasma to a reaction zone on the testsubstrate. Usually, the absorptive element will have a plasma capacitywhich is less than the amount of plasma in the whole blood volume, thusassuring that a precisely measured volume of plasma is transferred tothe test substrate. By then removing the filter member, plasma can beeluted from the absorptive element without interference from theseparated cellular components and excess whole blood.

According to a second method of the present invention, metered amountsof plasma are transferred from unmeasured blood samples to a testsubstrate by applying a volume of whole blood to an absorptive elementthrough a filter member. The absorptive element has a plasma capacitywhich is less than the amount of plasma in the whole blood volume sothat the absorptive element becomes saturated with a predeterminedamount of plasma. The filter member permits the passage of plasma andplasma macromolecules but inhibits the passage of blood cells, andexcess blood and blood cells are thus retained on the filter membrane.By removing the filter member from the absorptive element, excess bloodand blood cells are separated from the precisely measured amount ofplasma absorbed in the absorptive element. Optionally, an elution mediummay be applied to the absorptive element after the filter membrane hasbeen removed so that plasma absorbed in the absorptive element is elutedto a reaction zone on a fluid-receiving surface of the test substrate.

According to a third method of the present invention, measured amount ofplasma are transferred from unmeasured blood samples to a test substrateby applying a volume of whole blood to an absorptive element through afilter membrane. The filter membrane permits the passage of plasma andplasma macromolecules, but inhibits the passage of blood cells so thatblood cells are retained on the filter member. The filter member isremoved from the absorptive element after a time selected to permit thetransfer of a preselected amount of plasma to be absorbed in theabsorptive element. Usually, the filter member is removed before theabsorptive element becomes saturated with plasma. Optionally, an elutionmedium is applied to the absorptive element after the filter member hasbeen removed so the plasma absorbed in the absorptive element is elutedto a reaction zone on a fluid-receiving surface of the test substrate.

According to a fourth method of the present invention, measured amountsof plasma are transferred from whole blood to a test substrate by firstdrawing blood through a capillary tube into a reservoir in anapplicator. A filter membrane extending across a transfer port in theapplicator is then contacted against an absorptive element on the testsubstrate, where the absorptive element has a plasma capacity which isless than the amount of plasma in the blood reservoir. The filter memberpermits the passage of plasma and plasma macromolecules but inhibits thepassage of blood cells so that excess blood and blood cells are retainedon the filter member. The filter member is removed from the absorptiveelement after the absorptive element has become saturated with apredetermined amount of plasma.

According to the fifth method of the present invention, measured amountsof plasma are transferred from whole blood to a test substrate by firstdrawing blood through a needle inserted into a collection device filledwith patient blood, e.g. a vacuum blood collection device, into areservoir in an applicator. A filter membrane extending across atransfer port in the applicator is then contacted against an absorptiveelement on the test substrate, where the absorptive element has a plasmacapacity which is less than the amount of plasma in the blood reservoir.The filter member permits the passage of plasma and plasmamacromolecules but inhibits the passage of blood cells so that excessblood and blood cells are retained on the filter member. The filtermember is removed from the absorptive element after the absorptiveelement has become saturated with a predetermined amount of plasma.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a test system comprising an applicatorand a test substrate constructed in accordance with the principles ofthe present invention.

FIG. 2 is an exploded view of the applicator of the system of FIG. 1.

FIG. 3 is a cross-sectional view of the applicator of the system of FIG.1.

FIGS. 4A 4B, 4C, 4D, 4E, 4F and 4G illustrates alternative embodimentsof the test substrate and plasma absorptive element of the system ofFIG. 1.

FIGS. 5A 5B and 5C illustrate use of the system of FIG. 1 intransferring a measured amount of plasma to the test substrate andsubsequently eluting the plasma from absorptive element on the substrateto an adjacent fluid-receiving surface.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention includes a system and method for providing plasma,usually in precisely measured volumes, for subsequent analysis for thedetermination of target markers, analytes, enzyme activities, proteins,small molecules, infectious organisms, drugs, and the like. Plasmaconsists of whole blood from which the blood cells, including both redblood cells and white blood cells, have been separated. Generally, bloodproteins and other macromolecules, including the clotting proteins,immunoglobullins, and other blood factors and non-cellular components,remain in the plasma. In particular, the present invention provides forfiltration of cellular components from whole blood to provide plasma aswill be described in more detail hereinbelow. All blood componentssubstantially smaller than the cellular components will remain in theseparated plasma. Thus, virtually any test which can be performed onplasma can utilize plasma obtained by the systems and methods describedbelow.

Separation of cellular components from whole blood according to thepresent invention will generally be obtained by two-stage filtrationwith a chromatographic filter element in series with a size-exclusionfilter element. The chromatographic filter element will typically be adepth filter, often including two or more separate layers. Thechromatographic filter element performs an initial separation ofcellular components from the plasma based on chromatographic principles,i.e. the plasma will travel through the chromatographic element at arate faster than the blood cells which, because of their size arepreferentially slowed. The chromatographic filter element can becomposed of any conventional filter medium, such as glass fiber,polyester fiber, plastic fiber, metal fiber, composite cellulose andsynthetic fiber, nylon mesh, polyester mesh, synthetic fabric, and thelike. A preferred chromatographic filter element material is glassfiber. The area and depth of the chromatographic filter element willdepend on the volume of plasma to be separated from the applied wholeblood, as described in more detail below. Typically, the volume of thechromatographic filter element will be in the range from 25 mm³,preferably from 25 mm³ to 700 mm³. The depth of the chromatographicfilter element will typically be in the range from 1 mm to 10 mm,usually from 2 mm to 6 mm.

The size-exclusion filter membrane will have a pore size selected toexclude the passage of cells, typically being less than 7 μm, preferablybeing in the range from 0.1 μm to 7 μm, more preferably being from 0.4μm to 3 μm. The size-exclusive membrane can be composed of anyconventional membrane material, such as nitrocellulose, celluloseacetate, regenerated cellulose, polycarbonate, polyamide, and the like.A preferred size-exclusion filter membrane is polycarbonate.

The chromatographic filter element will have a volume which issufficiently large to retain blood cells and prevent blockage of thesize-exclusion membrane so that a desired volume of plasma can passthrough the combined filter member. Usually, the plasma capacity of thefilter member will be larger than that of the absorptive element on thetest substrate, as described in detail below in connection with certainembodiments of the test system. In some cases, however, such as whenplasma measurement is not being performed or plasma measurement is beingperformed by timed plasma transfer techniques, the plasma capacity ofthe filter member could be smaller than the plasma capacity of theabsorptive element.

Materials and constructions for two-stage filter systems suitable foruse in the present invention are described in U.S. Pat. No. 3,663,374,the full disclosure of which is incorporated herein by reference.

Plasma passing through the filter member is transferred to an absorptiveelement on a plasma-receiving substrate. The absorptive element can beany material or structure which is capable of absorbing the plasma bycapillary action. In a first exemplary embodiment, the absorptiveelement will be composed of a fiber capillary matrix, such as cellulosepaper, cellulose acetate, nitrocellulose, regenerated cellulose,polyamide, hydrophilic-treated polyethylene, nylon mesh,hydrophilic-treated polyester mesh, and the like. Indeed, virtually anyliquid-absorptive material which can absorb the plasma withoutdeleterious interaction with the components to be measured might finduse.

Alternatively, the absorptive element can be defined by a moldedcapillary matrix, such as those illustrated in FIGS. 4A-4G and describedin more detail hereinafter. Such molded structures provide for a veryprecise capillary volume to enhance the accuracy and repeatability ofplasma measurement performed by the present invention.

The absorptive element of the present invention will frequently be freeof any reactive components which would react or otherwise interact withthe target markers, analytes, or other components in the plasma beingtransferred. In such cases, analysis of the plasma will be performed byeluting or otherwise removing the plasma to an adjacent reaction zone asdescribed hereinafter. Alternatively, analysis could be performed bysubsequently applying test reagents directly to the absorptive elementand reading the results on the element itself. The latter protocol,however, is generally less preferred since the absorptive element willusually be saturated with plasma, making it difficult to apply furthertest reagents.

The test substrate will frequently include other components or elementsintended to provide or be part of the analysis to be performed. Forexample, the test substrate may comprise a fluid-receiving surface,where the absorptive element is located on said surface. Application ofan elution medium to the absorptive element can then cause flow of theplasma (diluted with the elution medium) onto the fluid-receivingsurface. Analysis can then be performed by providing reaction zones onthe fluid-receiving surface or by further transferring the separatedplasma remotely located reaction zone(s). The reaction zones willfrequently provide specific binding reagents capable of capturing atarget analyte or marker in the plasma. Optionally, the elution mediummay contain other components of a test system in order to perform adesired analysis.

The applicator of the present invention includes a transfer port and ablood-receiving reservoir adjacent the filter member. The transfer portwill have an area in the range from 15 mm² to 200 mm², preferably beingfrom 25 mm² to 100 mm², and usually although not necessarily having acircular periphery. The volume of the reservoir will typically be in therange from 50 μl to 500 μl, preferably from 100 μl to 400 μl. The volumeincludes the blood and plasma capacity of the filter member. That is,the applicator will have a total blood and plasma capacity which is thetotal of that present in the filter member (prior to transfer to theplasma-receiving test substrate) and that present in the "empty" volumeof the reservoir (i.e. the volume which is not occupied by filterelement). As plasma is drawn from the filter element to the absorptiveelement on the test substrate, it will be appreciated that blood in theempty volume of the reservoir will pass into the filter element, whereplasma is separated by the filter element components as described above.The total amount of plasma available to be transferred to the absorptiveelement, however, will be the total of that initially present both inthe filter element and the empty volume of the reservoir.

Referring now to FIGS. 1-3, a test system 10 comprises an applicator 12and a test substrate 14. The applicator 12 includes a casing 16 which istypically formed from molded plastic, such as polystyrene. Casing 16 hasan axial blood passage 18, an open blood reservoir 20, and a three-layerfilter element 22 (FIG. 3). The filter element 22 typically comprisesthree layers of glass fiber. Filter membrane 24 defines a lower contactsurface for the applicator, as described in more detail hereinafter. Theassembly is held together by ring 26, and a capillary tube 28, typicallya hypotube suitable for obtaining a blood sample by finger prick, ismounted next to the axial blood passage 18.

The test substrate 14 is illustrated as a disc 40 having absorptiveelement 42 at its center and a plurality of reaction zones 44 spacedradially outward from the absorptive element. The absorptive element 42is shown as a fiber matrix, such as a cellulose acetate disk. Thereaction zones 44 may conveniently be a specific binding substance, suchas an antibody, biotin, avidin, ligand, receptor, or the like. The disk40 may be composed of a variety of materials, such as acrylic or opticalquality polystyrene. The coupling of specific binding proteins and othersubstances to such solid phase materials is well described in thescientific and patent literature.

The reaction zone 44 will contain diffusively or non-diffusively boundreagent(s) that can react directly or indirectly with the sample analyteor a specific reaction product of the sample analyte. The reaction zonecan thus contain any assay reagent(s), including but not limited to, onemember of a binding pair such as an antibody, lectin, avidin,strepavidin or biotin, or may include components of a signal producingsystem or a combination of reagents to carry out assay methods. Thereaction zone will ultimately produce or cause to produce a specificsignal in response to a sample analyte. The signal is proportionaldirectly or inversely to the sample analyte concentration.

The disk 40 will be particularly suitable for use in interferometricassays, such as those described in co-pending PCT ApplicationUS94/07184, (which designates the United States) the full disclosure ofwhich is incorporated herein by reference.

A test substrate 50 having an alternative construction is illustrated inFIG. 4A. Test substrate 50 is a disk having a molded capillary matrix 52disposed at its center. The molded capillary element 52 comprises anarray of molded pyramidal protrusions formed over a generally flatplane. The interstices between the protrusions have capillary dimensionswhich are able to absorb a precise volume of plasma from the applicator12 of the system of FIG. 1. Test substrate 50 further includes aplurality of concentric walls 54, 56, and 58. These walls defineplurality of annular regions. Region 53 defined wall 54 is the plasmaelution and mixing region. After a measured volume of plasma istransferred to the capillary element 52, the plasma is washed intoregion 53. Additional reagent(s) may be added to the region 53 and mixedwith the plasma. The plasma and optional reagent(s) are then transferredto a reaction region 55 defined by concentric wall 56. Region 55 willinclude a plurality of spaced-apart reaction zones 59 similar to zones44 described with reference to FIG. 1. After the reaction is complete,the plasma may be transferred to a waste collection region 57 defined bywall 58.

Alternative capillary structures which could be employed on the testsubstrate 40 or 50 are illustrated in FIGS. 4B-4G. Such capillarystructures include a parallel capillary grating (FIGS. 4B and 4C), auniform checkerboard pattern capillary grating (FIGS. 4D and 4E), and acheckerboard pattern capillary grating having a central area with areduced capillary density (FIGS. 4F and 4G). The reduced capillarydensity area of the capillary structure of FIGS. 4F and 4G will be sizedand configured to engage the plasma transfer port of the applicator. Thearea of reduced capillary density permits withdrawal of the applicationfrom the capillary structure without substantial loss of transferredfluid sample. Surface tension and liquid cohesion within the filtermatrix of the applicator can cause fluid sample to be partiallywithdrawn from the capillary grating as the applicator is disengaged.Reducing the density of the capillary grating in the region of thedirect contact with the filter reduces direct fluid-to-fluid contact anddecreases the loss of precision associated with such sample fluid loss.

Referring now to FIGS. 5A-5C, use of the applicator 12 in transferring aplasma sample to an absorptive element 42 on test substrate 40 will bedescribed. The applicator 12 is filled with blood, typically by fingerprick with the capillary tube 28 (FIG. 5A). After the blood has beenobtained, applicator 12 is pressed against absorptive element 42 of testsubstrate 40 by engaging the crowned filter membrane 24 against anupper, plasma-receiving surface of the element 42. The applicator willbe pressed against the absorptive element 42 (as illustrated in FIG. 5B)until the desired volume of plasma has been transferred. In a preferredembodiment, the contact will be maintained until the absorbeddevelopment 42 is saturated with plasma, and the applicator 12 can beremoved from the test substrate at any time after saturation has beenachieved. In an alternative embodiment, the absorptive element 42 willhave a plasma capacity which may be greater than that of the plasmacarried in the applicator 12. In that case, the applicator 12 will beremoved after a time which is selected to permit transfer of a fixedamount of plasma to the absorptive element 42. Generally, such timetransfers will be less desirable since they will be less accurate. Inboth cases, the crowned profile of the filter membrane 24 isadvantageous since it helps assume continuous contact between themembrane and the absorptive element 42 as the filter element 22 iscompressed. Alternatively, or additionally, the contact surface of theabsorptive element 42 could also be crowned to promote good contact withthe filter membrane 24.

After the applicator 12 is removed, as shown in FIG. 5C, an elutionmedium may be applied to the absorptive element 42, resulting in theoutward flow of eluant from the element 42 to the plasma-receivingsurface of the substrate 40. As shown in FIG. 1, reaction zones willtypically be present on the substrate 40, allowing reaction with theeluant to perform a desired assay or test. Alternatively, the plasmacould be removed from the absorptive element 42 (or a capillaryequivalent as illustrated in FIGS. 4A-4G) by centrifugation, i.e.spinning the substrate sufficiently rapidly to cause the plasma to flowradially outward from the absorptive element due to centrifugal forces.Additional reagents may be added as necessary to perform the desiredassay or test.

Although the foregoing invention has been described in some detail byway of illustration and example, for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

What is claimed is:
 1. A method for transferring measured amounts ofplasma from unmeasured blood samples to a test substrate, said methodcomprisingapplying a volume of whole blood to an absorptive elementthrough a filter member, said absorptive element on a surface of a testsubstrate, wherein (a) the filter member permits the passage of plasmaand plasma macromolecules but inhibits the passage of blood cells and(b) blood cells are retained on the filter member; removing the filtermember from the absorptive element to separate excess blood and bloodcells from the measured amount of plasma absorbed in the absorptiveelement; and transferring plasma from the absorptive element to areaction zone on the surface of the test substrate.
 2. A method as inclaim 1, wherein the plasma is transferred from the absorptive elementto the reaction zone by applying an elution medium to the absorptiveelement.
 3. A method as in claim 1, wherein the plasma is transferredfrom the absorptive element to the reaction zone by spinning theabsorption element zone to cause the plasma to flow radially outward bycentrifugal force.
 4. A method as in claim 2, wherein the absorptiveelement has a plasma capacity which is less than the amount of plasma inthe whole blood volume and excess blood is retained on the filtermember, further comprising removing the filter member from theabsorptive element after applying the blood but before applying theelution medium.
 5. A method as in claim 1, further comprising removingthe filter member from the absorptive element after a time selected topermit the transfer of a preselected amount of plasma to the absorptiveelement.
 6. A method for transferring measured amounts of plasma fromunmeasured blood samples to an absorptive element on a test substrate,said method comprising:applying a volume of whole blood to an absorptiveelement through a filter member, wherein (a) the absorptive element hasa plasma capacity which is less than the amount of plasma in the wholeblood volume so that said absorptive element becomes saturated with apredetermined amount of plasma, (b) the filter member permits thepassage of plasma and plasma macromolecules but inhibits the passage ofblood cells, and (c) excess blood and blood cells are retained on thefilter member; and removing the filter member from the absorptiveelement to separate excess blood and blood cells from the measuredamount of plasma absorbed in the absorptive element.
 7. A method as inclaim 6, wherein the filter member is removed from the absorptiveelement after the absorptive element has become saturated with plasma.8. A method as in claim 7, further comprising applying an elution mediumto the absorptive element after the filter member has been removed,wherein plasma absorbed in the absorptive element is transferred to thereaction zone on a fluid-receiving surface of the test substrate.
 9. Amethod as in claim 7, further comprising, spinning the absorptiveelement after the filter member has been removed, wherein plasmaabsorbed in the absorptive element flows radially outward to a reactionzone on a fluid-receiving surface of the test substrate.
 10. A methodfor transferring measured amounts of plasma from unmeasured bloodsamples to an absorptive element on a test substrate, said methodcomprising:applying a volume of whole blood to an absorptive elementthrough a filter member, wherein (a) the filter member permits thepassage of plasma and plasma macromolecules but inhibits the passage ofblood cells, and (b) blood cells are retained on the filter member; andremoving the filter member from the absorptive element to separateexcess blood and blood cells from the measured amount of plasma absorbedin the absorptive element.
 11. A method as in claim 10, furthercomprising spinning the absorptive element after the filter member hasbeen removed, wherein plasma absorbed in the absorptive element flowsradially outward to a reaction zone on a fluid-receiving surface of thetest substrate.
 12. A method as in claim 10, wherein the filter memberis removed from the absorptive element before the absorptive element hasbecome saturated with plasma.
 13. A method as in claim 12, furthercomprising applying an elution medium to the absorptive element afterthe filter member has been removed, wherein plasma absorbed in theabsorptive element is transferred to a reaction zone on afluid-receiving surface of the test substrate.
 14. A method fortransferring measured amounts of plasma from whole blood to a testsubstrate, said method comprising:drawing blood through a capillary tubeinto a reservoir in an applicator; contacting a filter member extendingacross a transfer port in the applicator adjacent to the reservoiragainst a contact surface on an absorptive element on the testsubstrate, wherein (a) the absorptive element has a plasma capacitywhich is less than the amount of plasma in the blood, in the reservoir,(b) the filter member permits the passage of plasma and plasmamacromolecules but inhibits the passage of blood cells, and (c) excessblood and blood cells are retained on the filter member; and removingthe filter member from the absorptive element to separate excess bloodand blood cells from the measured amount of plasma absorbed in theabsorptive element.
 15. A method as in claim 14, wherein the blood isdrawn via a finger prick.
 16. A method as in claim 14, wherein the bloodis drawn from a vacuum blood collection device.